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my $is_number = qr/^
                   [+-]?                             # optional sign
...
                  )
                   ([eE][+-]?\d+)?                   # optional expone
+nt
                   $/xo;

#!/usr/bin/perl

...
}

exit;

-123.004: true
-0.008: true
...
12345: true
d: false
test: false