Actually Perl DWIMs properly here, and /^\s+$/ catches empty lines. The $ is quite magic, if the \n at the end of the string/line is matched then $ matches the empty end-of-string, if the \n is not matched then $ matches it, plus the end-of-string.

You can test it with this code:

my $string="12\n";
print 'matches ^\d+\n$' . "\n" if( $string=~ /^\d+\n$/);
print 'matches ^\d+$' . "\n"   if( $string=~ /^\d+$/);
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