You really need to clarify your definitions 'string' and 'octal' through some examples. Octal is always a 'string' representation of a number. Ie. It uses a string of ascii/unicode characters for the digit '0' to '7' to represent the actual number.

Assuming the input received by the Perl program looks something like this:

C:\test>perl -le" print $ARGV[ 0 ] " 0666
0666
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Then all you need to do is tell Perl to iterprete the contents of that string as octal (intuatively using the oct built-in function), and Perl will take care of the rest for you:

C:\test>perl -le"my $n = oct( $ARGV[ 0 ] ); printf qq[decimal %d (as o
+ctal: %o)\n], $n, $n " 0666
decimal 438 (as octal: 666)
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