Why would your users have to type the double quotes? 
#!/usr/bin/perl -w
use strict;
my $substitution = '$line =~ '.<DATA>;

my $line = "abcdef";
eval $substitution;
print $line;
__DATA__
s/abc(\w+)f/xxx$1x/;

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output: 
xxxdex

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Update: what I mean is; the problem you're describing only occurs in string literals (and it could already be a lot less annoying if you used single quotes). this interpolation does not occur in strings per se.
"What should it profit a man, if he should win a flame war, yet lose his cool?"

In reply to Re^3: User configurable regex by Joost
in thread User configurable regex by MiggyMan

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