#!/usr/bin/perl
...
use threads;

....
my($thr) = threads->new(\&showpic);
....
    


sub showpic { 
        system "eog $picfilename";
}

Can't locate auto/threads/kill.al in @INC (@INC contains: /etc/perl /usr/local/lib/perl/5.8.8 /usr/local/share/perl/5.8.8 /usr/lib/perl5 /usr/share/perl5 /usr/lib/perl/5.8 /usr/share/perl/5.8 /usr/local/lib/site_perl .) at <snip>

use strict;
use warnings;

[...]

my $pid = fork;
die "Failed to fork: $!\n" unless (defined $pid);
unless ($pid) {
  exec '/path/to/program', 'argument';
  die "Failed to exec: $!\n";
}

[...]

{
  local $SIG{CHLD} = 'IGNORE';
  kill TERM => $pid or die "Failed to kill $pid: $!\n";
}