$/ = undef;
open( FILE, "somescript.pl" ) || die "can't open: $!"
$script = <FILE>
close( FILE );

$ARGV[0] = "Whatever $DECRYPT_WORK_PATH$PLAINTEXT_FILENAME IS";
eval $script;
if( $@ ) {
  print STDERR "Error: yada yada\n";
}

##</code><code>##

$ARGV[0] = "whatever";
do 'somescript.pl'