Not solving your problem but you could avoid the @{[join "|", @pairs]} by using the fact that regexen act like double quotes when interpolating arrays and changing the default list separator ($").

my $matcher = do
   {
       local $" = q{|};
       qr{(?=(@pairs))};
   };
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Also, you can avoid having to escape the double quotes in your print statements when you want to interpolate by using quoting constructs (qq{...} for double quotes).

$ perl -e '
> @arr = qw{a b c};
> print qq{"@arr"\n};'
"a b c"
$
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I hope these points are useful.

Cheers,

JohnGG