use POSIX qw(strtod);
use strict;

foreach (undef, qw(4 0 fish)) {
    print $_ , " is numeric? ", is_numeric($_), "\n";
}

sub getnum {
    my $str = shift;
    $str =~ s/^\s+//; $str =~ s/\s+$//; $! = 0;
    my($num, $unparsed) = strtod($str);
    if (($str eq '') || ($unparsed != 0) || $!) {return;} else {return $num;} 
} 

sub is_numeric { defined scalar &getnum }