in reply to Shorten a list...

One way is to keep track of the group count in a hash: 
use warnings;
use strict;

my %groups;
while (<DATA>) {
    next unless /\S/;
    chomp;
    $groups{$_}++;
}

for (sort keys %groups) {
    print "$_\n" if $groups{$_} > 1;
}

__DATA__
Group 1

Group 2 

Group 3
Group 3

Group 4

Group 5 
Group 5 
Group 5

[download]
Prints... 
Group 3
Group 5

[download]

Replies are listed 'Best First'.
Re^2: Shorten a list...
by la (Novice) on Oct 16, 2011 at 00:18 UTC

    Thanks for the quick reply. I want the resultant list to read:

    Group 3
    Group 3
    Group 5
    Group 5
    Group 5

    is this possible with the code you have given?

[reply]
      You're welcome.

      The code I provided can be modified to produce that output. Give it a try.

[reply]

In Section Seekers of Perl Wisdom