Bugs:

• Missing comma
• "-oG" and "pentest.txt" are separate args. You can't pass them as one.
• The dash should be the start of the arg, not a space.

You also introduced four bugs in the error handling!

Fixed:

system( 'nmap', $ver, $os, $target, '-oG', 'pentest.txt' )
   or die "nmap failed" $!/$?\n";
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