my %extToLang=(
    pl => 'perl',
    pm => 'perl',
    c => 'c',
    h => 'c',
    sh => 'bash',
    bash => 'bash',
);
my $filename;
my $lang;

# setup $filename and perhaps also $lang

if (!$lang and ($filename=~/\.([^.]+)$/) {
    $lang=$extToLang{$1};
}
$lang or die "Missing language parameter";