i have tried your code and works fine.. there is lots of whitespace but other than that.. works fine..

#!/usr/bin/perl -w
use strict;
use LWP::UserAgent;

sub retrieve_url
{
  my $ua = LWP::UserAgent->new;
  my $response = $ua->get(shift);
  if ($response->is_success)
  { return $response->content;}
  else 
  {die $response->status_line;}
}

my $var = retrieve_url('http://www.finasta.lt');

print $var;
[download]

maybe somewhere in your script you maybe prematurely cropping out the variable you are inserting the content into..