Do it this way

tachyon

my $var = "my dog spot";
my $expr = "(my dog) spot";
$var =~ m/$expr/;
my $sub = "$1 spooge";
$var =~ s/$expr/$sub/;
print $var;
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By matching first we define $1. When we define $sub $1 is interpolated into the string. We then do the substitution with the replacement string fully formed, and ta da - it works

Cheers