parser $_;

##</code><code>##

parser($_);

##</code><code>##

use strict;
use warnings;

sub parser {
    print "Hello, world\n";
}
open INFILE, '<', $0;

while (<INFILE>) {
    chomp;
    parser $_;
}

__END__

##</code><code>##

use strict;
use warnings;

sub parser;

open INFILE, '<', $0;

while (<INFILE>) {
    chomp;
    parser $_;
}

sub parser {
    print "Hello, world\n";
}

__END__