Looks like Number::Format would be an easy way to go. I haven't used it myself, but by my reading of the documentation, it would be as simple as:

use Number::Format ':subs';

for (qw($100000000.00 $100.00 $1000.00)) {
    s/^\$//;
    $_ = '$' . format_number( $_, 2, 2 );
    print;
}
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It's even easier if there weren't the dollar sign at the front.