You might find this version of the OP code useful--or not:)

It uses tasklist.exe to get the memory stat rather than Win32::API as I have had great problems with using that module with threads in the past and despite several attempts to work out why, my XS foo isn't strong enough.

#! perl -slw
use strict;
use threads;

sub mem {
    my( $usage ) =  `tasklist /NH /FI \"pid eq $$\" ` =~ m[ (\S+) \s+ 
+K \s* $ ]x;
    return $usage;
}

sub threadTest{ sleep 1 }

my $startmem = mem;
print "StartMem: $startmem KB";

for my $pass ( 1 .. 50 ) {
    my @threads = map{
        Win32::Sleep 100;
        threads->new(\&threadTest);
    } 1 .. 25;

    my $midmem = mem;

    $_->join for @threads;
    undef @threads;

    my $endmem = mem;
    print "Pass $pass\tMidMem: $midmem KB\tEndMem: $endmem KB";
}
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