yeah sorry I wasn't able to test from Android, the problem is that the substitution part of s/// is supposed to be a literal. Otherwise you have to apply the /e eval modifiers.

my $string = 'Hello sailor';
my $regex = 'Hello (.*)';
# my $substitution = 'Goodbye \1';

$string =~ s/$regex/Goodbye $1/;
print "string is now <$string> \n";
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/usr/bin/perl -w /tmp/regex.pl 
string is now <Goodbye sailor>
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Please note that \1 is somehow deprecated in the substitution part and will cause warnings.

Its only legitimate use is in the match part for backreference.