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sub foo {
    my ($arg1, $arg2) = @_;
    # INSERT CODE VALIDATION HERE
    ...
}

sub foo {
    my ($arg1, $arg2) = @_;
    defined($arg1) or die "arg1 is required"; $arg1 =~ qr/\A\w+\z/ or 
+die "arg1 must be alphanums only"; defined($arg2) or die "arg2 is req
+uired"; # INSERT CODE VALIDATION HERE
    ...
}