No, not undefined. If the \d in /(\d*)/ matched zero times, $1 would be the empty string.

You found the problem (since the OP is having problems with an empty string), it's just the description that's flawed.

>perl -we"my $x = '' < 2"
Argument "" isn't numeric in numeric lt (<) at -e line 1.

>perl -we"my $x = undef() < 2"
Use of uninitialized value in numeric lt (<) at -e line 1.
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