Quite frankly, I think that in most instances, it is simple better to allow Perl to detect these kind of trivial errors, It most times does a better job of reporting the nature of the problem than the programmer will:

#! perl -slw
use strict;

sub times7 {
    return $_[ 0 ] * 7;
}

print "times7( $_ ) returned:", times7( $_ ) for 1, 2, 3, 0, undef, 'f
+red';
__END__
P:\test>junk
times7( 1 ) returned:7
times7( 2 ) returned:14
times7( 3 ) returned:21
times7( 0 ) returned:0
Use of uninitialized value in concatenation (.) or string at P:\test\j
+unk.pl line 8.
Use of uninitialized value in multiplication (*) at P:\test\junk.pl li
+ne 5.
times7(  ) returned:0
Argument "fred" isn't numeric in multiplication (*) at P:\test\junk.pl
+ line 5.
times7( fred ) returned:0
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