for my $input (qw<
    +4
    1.0
    1e5
    12345678901234567890
    1.234567890123456789
>) {
    if( $input+0 eq $input ) {
        print "$input is a number!\n";
    } else {
        print "$input is not ", 0+$input, "\n";
    }
}
__END__
+4 is not 4
1.0 is not 1
1e5 is not 100000
12345678901234567890 is not 1.23456789012346e+019
1.234567890123456789 is not 1.23456789012346