in reply to I don't understand.
in thread Detecting if a scalar has a number or string

Among other reasons: 

sub is_a_num { $_[0] =~ m/^[0-9]*$/ }
my $not_at_all = '';
print is_a_num( $not_at_all ) ? "Yeppers.\n" : "Nopers.\n";

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Beware the Star of Regex!

In Section Seekers of Perl Wisdom