$s = 'blah blah, blah blah blah (blah, blah, blah), blah, blah';;

$s =~ s[ ( \( [^)]+ \) ) ][ ( my $x = $1 ) =~ tr[,][;]; $x ]gex;;

print $s;;
blah blah, blah blah blah (blah; blah; blah), blah, blah