input	oct result	Comment
107	71	Octal
108	108	Decimal
109	109	Decimal
10a	266	Hexadecimal
10f	271	Hexadecimal
10g	8	Octal, ignoring 'g'

Does that seem anyway sensible or useful to you? Or perhaps oct 108 should return 89 (because it's base 9?). And should oct 45 be 29 (because it's base 6)?.

Abigail

No, the result of oct("108") is "8" but I think it should be undef.

How do your statements: No, I didn't give oct a valid hex number, I gave it an invalid octal number which could have been understood as an otherwise valid decimal number. and No, the result of oct("108") is "8" but I think it should be undef. relate to each other? 108 is an otherwise valid decimal number. You also seem to be defending, or at least sharing, the OP's ideas. And the OP wants to have a function that given decimal input, returns the decimal representation - so oct 108 (or whatever name this function has) should return 108.

If you think that oct 108 should be undefined, and not 8, do you also think that $x = "10 dogs" + "5 chickens" should result in $x being undefined, and not 15? What about

print "Please enter a number: ";
my $num = <>;
printf "Its square is %d.\n" => $num * $num if $num =~ /^\d+$/;
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Should that print "Its square is 0.", after issueing a warning?

Abigail