#!/usr/bin/perl -w
use strict;
use warnings;

while (<DATA>) {
    print "start of loop\n";
    print "\$_ = \"$_\"\n";
}

__DATA__
<a>a,
b</a>
<b>a,b</b>
<b>a,b
</b>

##</code><code>##

#!/usr/bin/perl -w
use strict;
use warnings;

{
    local $/;
    while (<DATA>) {
        print "start of loop\n";
        print "\$_ = \"$_\"\n";
    }
}

__DATA__
<a>a,
b</a>
<b>a,b</b>
<b>a,b
</b>

##</code><code>##

#!/usr/bin/perl -w
use strict;
use warnings;

my $var = do { local $/; <DATA> };
print "\$var = \"$var\"";

__DATA__
<a>a,
b</a>
<b>a,b</b>
<b>a,b
</b>

##</code><code>##

$var =~ s!<a>(.*?),(.*?)</a>!<a>$1A$2</a>!gs;

##</code><code>##

__DATA__
This is to test.
<a>a,
b</a>
<b>a,b</b>
<b>a,b
</b>
<a>ab</a>
<c>a,b</c>
<a>a,b</a>