The mehod suggested by astaines works but the script he gives does not because perl only warns about adding to non-numeric variables, so this is not caught by the eval. It is therefore necessary to make warnings fatal within the eval block:

sub is_number {
    my $test = shift;

    eval {
        local $SIG{__WARN__} = sub {die $_[0]};
        $test += 0;
    };
    if ($@) {return 0;} else {return 1;} 
}
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