Using eval, you can achieve your need,

$_=q{nell'amaca};
my $search=q{nell'([[:alpha:]]+a)};
my $subst='na $1';

$expr = "s/$search/".eval $subst."/g";
eval $expr;
print;
print "\n";
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First we are constructing the substitution, in that using the eval, the substitute pattern is evaluated. And then the total substitution has been evaluated.

I hope this would match your requirement.