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#!/usr/bin/perl

...
print "num:$num let:$let\n";

__END__

num:aa let:aa

Argument "z" isn't numeric in addition (+) at C:\temp\foo.pl line 9.

C:\temp>perl -le "$a='z';print++$a"
aa

C:\temp>perl -le "$a='9';print++$a"
10

C:\temp>perl -MO=Concise -e "$a='z'" >z
-e syntax OK
...
< 3        <$> const[PV "z"] s ->4
---
> 3        <$> const[PV "9"] s ->4

C:\temp>perl -le "$a='foo9';print++$a"
fop0

C:\temp>perl -le "$a='9bar';print++$a"
10