The answer btrott gave is ofcourse universal and good, but you can also do this (but only in Perl 5.6.0 I think): 
   print bin2dec(0b1101) . "\n";
   print dec2bin(13) . "\n";

   sub bin2dec {
       return(sprintf "%d",shift);
   }

   sub dec2bin {
       return(sprintf "%b",shift);
   }

[download]
Update: I forgot to mention that oct() can be used (in Perl 5.6.0) to convert from binary to decimal: 
   perl -e 'print oct("0b1101") . "\n";'

[download]

In reply to RE: decimal to binary by dempa
in thread decimal to binary by Punto

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