use strict;
use warnings;

my $str = "ABCDEFGHIJKLMNOP";
my $seed = "FGHI";
my $length = 7;

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

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

##</code><code>##

"ABCDEFGHIJKLMNOP" "ABC" 7 => ABCDEFG
"ABCDEFGHIJKLMNOP" "BC" 7 => ABCDEFG
"ABCDEFGHIJKLMNOP" "MN" 7 => JKLMNOP