Off the top of my head:

Get swfdump from SWFTOOLS (GPL)

# Windows syntax
my $size = `swfdump -X -Y "$qfn"`
   or die "...";
my ($x, $y) = $size =~ /^-X (\d+) -Y (\d+)$/
   or die "...";
[download]

I strongly recommend a solution that doesn't use an external program.

By the way, it's not called PERL. It's not called FLASH either.

Update: Added missing "= $size". Tested. Added recommendation.