my @pairs = ([0.456234, 'url 1'], [0.42323, 'url 2'], [0.456234, 'url 3'], [0.456234, 'url 4']);

my $max;
my @url;
foreach(@pairs) {
    if(!defined $max or $max < $_->[0]) {
        $max = $_->[0];
        @url = $_->[1];
    } elsif($max == $_->[0]) {   # caution: exact identity with floating point numbers is rare, so this might disappoint a little
        push @url, $_->[1];
    }
}

use Data::Dumper; print Dumper (\@url);