use strict;
use warnings;
use Data::Dumper;

my $token = 'perl';
my @contexts;
chomp( my @lines = <DATA> );

foreach my $line (@lines) {
	while ( $line =~ /\b $token \b/gip ) {

		my $before_string = ${^PREMATCH};
		my $after_string  = ${^POSTMATCH};

		if ( length $before_string > 5 ) {
			$before_string = substr( $before_string, -5 );
		}

		if ( length $after_string > 5 ) {
			$after_string = substr( $after_string, 0, 5 );
		}

		push @contexts, [ $before_string, $token, $after_string ];
	}
}

print Dumper \@contexts;

__DATA__
ABCDEFGHIJKLMNOP_PERL_ABCDCEFGHIJKLOMNP
0123456789ABCDEFGHIJKLMNOP_ PERL _ABCDCEFGHIJKLOMN_ PERL _P0123456789
ABCDEFGHIJKLMNOPPERLABCD PERL CEFGHIJKLOMNP
0123456789_ PERL _0123456789

##</code><code>##

$VAR1 = [
          [
            'MNOP_',
            'perl',
            '_ABCD'
          ],
          [
            'LOMN_',
            'perl',
            '_P012'
          ],
          [
            'LABCD',
            'perl',
            'CEFGH'
          ],
          [
            '6789_',
            'perl',
            '_0123'
          ]
        ];