$t1 is a variable. If it doesn't contain the right id value, the regex won't work.

I'd rather do the following instead:

my %fmt = $member_styles -> contents =~ /\bnumFmtId="(\d+)"\s+formatCo
+de="([^"]*)/g;
my $formatCode = $fmt{$t1} || ''
[download]

and this also gives you a chance to inspect which format codes it does recognize.