In your example $y is a string of characters, but when you eval it with "eval $y", eval expects that string of characters to be Perl code. You need to give eval actual code to eval, not just a string of characters. Do this by wrapping that string of characters in Perl code (in quotes):

my $x = 'foo';
my $y = 'The answer is $x';
my $z = eval enquote($y);

sub enquote {
    return 'qq{' . shift() . '}';
}
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There's no great reason to have used the enquote sub rather than just inlining the buildup of the Perl-quoted string other than to attempt to make it more clear what the intent is. I could have just done this:

my $z = eval 'qq{' . $y . '}';
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Or even...

my $z = eval "qq{$y}";
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In any case, the goal is to turn a raw string of characters into something that eval can reasonably compile as code that when evaluated returns the original string with interpolation in place.