use strict;
use warnings;
use List::Util qw/min/;

my $str = "ABCDEFGHIJKLMNOP";
my $seed = "FGH";
my $length = 7;
my $r_lim = 2;
my $l_lim = 1;

my $slen = length $seed;
my $ipos = index $str,$seed;

my $lexpand = $slen < $length ? ($length - $slen)/2 : 0;
$lexpand = min ($ipos, $l_lim, $lexpand);
my $rexpand = $length - $slen - $lexpand;
$rexpand = $r_lim < $rexpand ? $r_lim : $rexpand;
print +(substr $str,($ipos - $lexpand),($lexpand + $slen + $rexpand)),"\n";