Since my original reply, I have discovered that my concept of "build the command and evaluate it" can be generalized to meet your original requirement.

use strict;
use warnings;
my $regex=<STDIN>;                    #Entering s/\),\(/\)\n\(/gi 
chomp $regex;
open (INPUTFILE, "< $filein");
while (<INPUTFILE>) {
    my $line=$_;
    #$line =~ $regex;
    eval "\$line =~ $regex";
};
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