To debug problems like this, eliminate what doesn't matter until you find what does matter. This is the code you should have posted:

use strict;
use warnings;

use threads;

sub PINGIT {
    print "PINGIT\n";
    return 0;
}

sub THPING {
    print "THPING\n";
    my @th;
    for (1..5) {
        push @th, threads->create(\&PINGIT);
    }
    $_->join for @th;
    return 0;
}

{
    my %opts;

    my @th;
    for (1..2) {
        if ($ARGV[0]) {
            # Doesn't generate error message
            push @th, threads->create(\&THPING);          
        } else {
            # Generate error message
            push @th, threads->create(\&THPING, \%opts);
        }
    }

    $_->join for @th;
}
[download]

>perl 807726.pl 0
THPING
THPING
PINGIT
PINGIT
PINGIT
PINGIT
PINGIT
Scalars leaked: 1
Scalars leaked: 1
Scalars leaked: 1
PINGIT
PINGIT
PINGIT
Scalars leaked: 1
Scalars leaked: 1
Scalars leaked: 1
Scalars leaked: 1
PINGIT
PINGIT
Scalars leaked: 1
Scalars leaked: 1
Scalars leaked: 1

>perl 807726.pl 1
THPING
THPING
PINGIT
PINGIT
PINGIT
PINGIT
PINGIT
PINGIT
PINGIT
PINGIT
PINGIT
PINGIT
[download]

I don't know why that gives an error. It could very well be a bug in Perl. (See perlbug if you wish to submit a bug report.)

The problem is not specific to a version of Perl (reproduced on 5.8.0, 5.8.8, 5.10.0, 5.10.1) or to an OS (reproduced on WinXP, Linux).