Why do you append the .txt extension to $ins_filename but use $txt_filename in your system? Anyway:

($in_filename, $out_filename) = @ARGV;
$out_filename = $in_filename if ( !($out_filename) );
for my $num (1..20) { system ("platon", "-c", "$in_filename$num.txt") 
+}
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Note it's much better to use the multi-argument form of system. See perldoc -f system.

Makeshifts last the longest.