Have you looked at what the code returns?

It merely quotes a string. It could have been written as

my $perl5patt = "(??{join q{|}, $quotemeta \@{$var_name}})";
[download]

I don't know where that string will be used, but it looks like it will be used in a regular expression and will try to match one or more words that are stored in the variable named by $var_name.

Personally I think that it should be faster to (re)build the regular expression on each change of @$var_name, but I don't know what code you have that necessitates executing code when trying to match something. The documentation for the ??{} construct lives in perlre

There's no call to join in that call. You have a string constructor and an assignment.

Presuming from the (??{ }), the string will later be interpolated into a regex pattern, at which point a match against that pattern could call join, but it won't be the join you descrived. It will be

join q{|}, XXX \@{YYY}
[download]

where XXX is the contents of $quotemeta and YYY is contents of $var_name joined by spaces. Double quoted strings (qq{}) interpolate.

my $quotemeta = 'XXX';
my $var_name = 'YYY';
my $perl5patt = qq{(??{join q{|}, $quotemeta \@{$var_name}})};
print $perl5patt, "\n";
[download]

(??{join q{|}, XXX @{YYY}})
[download]

Since you didn't specify what $quotemeta and $var_name contain, we can't tell if the code executed by the regex engine will match join EXPR, LIST or not.