I now realize that I only saw half of the problem. The wanted idea seems to be double-interpolation of 'some$thing' into "someTHING" by replacing the string '$thing' within $regexp with the value of the variable $thing.

Your points are close but wrong - Perl allows stuff to appear in regular expressions after the $ metacharacter. Otherwise, Perl regular expressions act like double-quoted strings and hence interpolate only once:

#!perl -wl
use strict;
my $regexp = 'some$thing';
my $thing = 'THING';

my $target = 'this is some$thing strange.';

print $regexp;
print $target;

print '$target =~ /$regexp/ ', $target =~ /$regexp/;
print '$target =~ /\Q$regexp\E/ ', $target =~ /\Q$regexp\E/;
print '$target =~ /some\$thing/ ', $target =~ /some\$thing/;
print '$target =~ /some$/ ', $target =~ /some$/;
my $eol_in_the_middle = qr/some$(?:thing)/;
print 'eol_in_the_middle ',$eol_in_the_middle;
print '$target =~ /$eol_in_the_middle/ ',$target =~ /$eol_in_the_middl
+e/;
[download]

Of course, it's kinda hard to make $eol_in_the_middle match, but you can do it by using the /m switch and changing the RE and target string a bit:

my $eol_in_the_middle2 = qr/some$(?:\s*thing)/;
my $target2 = "this is some\nthing strange.";
print '$target2 =~ /$eol_in_the_middle2/ ',$target2 =~ /$eol_in_the_mi
+ddle2/sm;
[download]