The transliteration (tr) operator could also be used here to avoid the full overhead of regular expressions.

 
use strict;
use warnings;

my $grossAmt = "12,345,678.9";

$grossAmt =~ tr/,//d;
$grossAmt = sprintf('%.2f', $grossAmt);

print $grossAmt, $/;

__END__
output:
12345678.90
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