Let me give just one datapoint:

#!/usr/bin/perl

use strict;
use warnings;

undef $/;

my $code = <DATA>;

my $RUNS = 10;

my $no_threads = "/opt/perl/bin/perl-64";
my $threads = "/opt/perl/bin/thr-perl-64";

foreach my $perl ($no_threads, $threads) {
    my ($sum_r, $sum_u) = (0, 0);
    foreach my $runs (0 .. $RUNS) {
        my $res = `/usr/bin/time -p $perl -e '$code' 2>&1 `;
        next unless $runs;
        $sum_r += ($res =~ /real\s*(\d+\.\d+)/)[0];
        $sum_u += ($res =~ /user\s*(\d+\.\d+)/)[0];
    }
    printf "real: %.2f; user: %.2f\n", $sum_r / $RUNS, $sum_u / $RUNS;
}

__DATA__
use strict;
use warnings; 

foreach (1 .. 1000) {
    my @arr;
    foreach (1 .. 1000) {
        push @arr, 1;
    }
}
__END__
real: 0.43; user: 0.43
real: 0.54; user: 0.54
[download]

The short program, pushing '1' a thousand time on an array, takes about 25% more when threads are compiled in. Of course, YMMV.