You could use the substr method to get the first 3 letters and use a hash of arrays to store the matching strings. While probably not the most efficient, the following might be a good place for you to start:

#!/usr/bin/perl

use strict;
use Data::Dumper;

my (@words, $substr, %hash);

@words = qw/sem sema cat semok hgt cato seme cate/;

foreach (@words) {
    $substr = substr( $_, 0, 3 );
    push( @{ $hash{$substr} }, $_ );
}

print Dumper(\%hash);

exit;


$VAR1 = {
          'cat' => [
                     'cat',
                     'cato',
                     'cate'
                   ],
          'sem' => [
                     'sem',
                     'sema',
                     'semok',
                     'seme'
                   ],
          'hgt' => [
                     'hgt'
                   ]
        };
[download]

hope this helps,
davidj