If it acts just like a number as far as Perl is concerned, then it's a number as far as I'm concerned.

You can let Perl tell you what it thinks then...

#!perl

use strict;
use warnings;

sub seems_like_number {
    my $thing = shift;
    use warnings qw/FATAL all/; # Promote warnings to fatal, so
                                # they can be trapped.  The effect is
                                # lexically scoped.
    eval {
        $thing += 0;
    };
    return $@ ? 0 : 1;
}

while ( <DATA> ){
    chomp;
    if( seems_like_number( $_ ) ) {
        printf "%10s seems like a number.\n", $_;
    } else {
        printf "%10s doesn't seem like a number.\n", $_;
    }
}

__DATA__

1234
1.234
-2.345
1-1+2
0.032
.321
ASDF
ASDF1234
1234ASDF
[download]

The first line after __DATA__ is intentionally blank, to test whether or not an empty string will qualify as a number... and of course it won't.

Ok, now if Perl thinks it's not a number, you'll know about it, and if Perl doesn't object to it being considered a number, you'll know that too.