print "BEFORE:LINE = $line,EXPR= $expr,NEW_EXPR= $n_expr\n";
$line =~ s/$expr/$n_expr/;
print "AFTER:LINE = $line,EXPR= $expr,NEW_EXPR= $n_expr\n";

##</code><code>##

The o/p of above is:
BEFORE:LINE = input [`CLW:::0] abc;,EXPR = [`CLW:::0], NEW_EXPR= [$CLW:0]
AFTER:LINE = input [`CLW:::0] abc;,EXPR = [`CLW:::0], NEW_EXPR= [$CLW:0]