... problem: the single quotes around  $directory in your homebrew error message will prevent Perl from interpolating the var.

Not so. The scalar is interpolated within double-quotes; the single-quotes in the same string have no effect on interpolation.

>perl -wMstrict -le
"my $string = 'hi there';
 my $strung = qq{'$string'};
 print $strung;
"
'hi there'
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