I think the following code would be more consice:

my $logname = quotemeta($ENV{LOGNAME});
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In your version the regex does nothing since you don't use the results from it. quotemeta will automatically untaint the variable for you, so you don't need a regex at all. Perhaps it was a typo, and you meant to use quotemeta($1)? Not a big deal, but I thought it was worth mentioning.

Your explaination for why there was a problem is spot on though...