Perl variable $/ When the context is row pattern, $/ defines what to distinguish rows

By default, it is obvious that \n is used to distinguish lines, and \n is also called a newline character.
When reading a sequence and reading by line, the newline character is used as the standard.

The contents of the read file are as follows:
LOCUS JX118024 460 bp DNA linear PLN 25-SEP-2012
DEFINITION Fragaria vesca subsp. americana RNA polymerase beta subunit (rpoC1)
gene, partial cds; plastid.
/
ACCESSION JX118024
//
VERSION JX118024.1 GI:402238751
KEYWORDS .
how
///
SOURCE plastid Fragaria vesca subsp. americana

First example: default

Copy the codeThe code is as follows:

 
#!/bin/perl 
my $record =' '; 
open (DNAFILENAME,'f:\\perl\\')||die("can not open the file!"); 
$record = <DNAFILENAME>; 
print $record; 

This is the case without any changes, which means that one line is read by default, and the result is as follows:
F:\>perl\
LOCUS JX118024 460 bp DNA linear PLN 25-SEP-2012

If we change the value of $/, according to the characteristics of our file, we first change it to $/="///\n;

Copy the codeThe code is as follows:

 
#!/bin/perl 
my $record =' '; 
open (DNAFILENAME,'f:\\perl\\')||die("can not open the file!"); 
$/="///\n"; 
$record = <DNAFILENAME>; 
print $record; 

The results we get are as follows:
F:\>perl\
LOCUS JX118024 460 bp DNA linear PLN 25-SEP-2012
DEFINITION Fragaria vesca subsp. americana RNA polymerase beta subunit (rpoC1)
gene, partial cds; plastid.
/
ACCESSION JX118024
//
VERSION JX118024.1 GI:402238751
KEYWORDS .
how
///

We can see here that this line is based on // as the separator, and the entire part above is regarded as a line.

Also, not only characters can be used as delimiters, but also letters can be used. Add to us how as delimiters, $/="how\n";

Copy the codeThe code is as follows:

 
#!/bin/perl 
my $record =' '; 
open (DNAFILENAME,'f:\\perl\\')||die("can not open the file!"); 
$/="how\n"; 
$record = <DNAFILENAME>; 
print $record; 

The results are as follows:
C:\Documents and Settings\Administrator>f:perl\
LOCUS JX118024 460 bp DNA linear PLN 25-SEP-2012
DEFINITION Fragaria vesca subsp. americana RNA polymerase beta subunit (rpoC1)
gene, partial cds; plastid.
/
ACCESSION JX118024
//
VERSION JX118024.1 GI:402238751
KEYWORDS .
how

C:\Documents and Settings\Administrator>

Similarly, we can completely abandon lines in the traditional sense. For example, we use the fifth line of the example as the delimiter:

Copy the codeThe code is as follows:

 
#!/bin/perl 
my $record =' '; 
open (DNAFILENAME,'f:\\perl\\')||die("can not open the file!"); 
$/="ACCESSION"; 
$record = <DNAFILENAME>; 
print $record; 

The results are as follows:
F:\>perl\
LOCUS JX118024 460 bp DNA linear PLN 25-SEP-2012
DEFINITION Fragaria vesca subsp. americana RNA polymerase beta subunit (rpoC1)
gene, partial cds; plastid.
/
ACCESSION
F:\>

Let’s take another example: use /\n as the delimiter:

Copy the codeThe code is as follows:

 
#!/bin/perl 
my $record =' '; 
open (DNAFILENAME,'f:\\perl\\')||die("can not open the file!"); 
$/="/\n"; 
$record = <DNAFILENAME>; 
print $record; 

The result we expect should be that the content before the fourth line should be one line, but is this the result?
F:\>perl\
LOCUS JX118024 460 bp DNA linear PLN 25-SEP-2012
DEFINITION Fragaria vesca subsp. americana RNA polymerase beta subunit (rpoC1)
gene, partial cds; plastid.
/
ACCESSION JX118024
//

F:\>

Why didn't the first / match?

In fact, there is not just one / in this line here, but there are other components here. We completely delete this line, and then enter only one / again, and we will match it.
F:\>perl\
LOCUS JX118024 460 bp DNA linear PLN 25-SEP-2012
DEFINITION Fragaria vesca subsp. americana RNA polymerase beta subunit (rpoC1)
gene, partial cds; plastid.
/

F:\>
This time we got the correct result.