Thank you - that explains exactly where the problem is...

Would this be a sensible way to untaint $ENV{'PATH'} without losing the portability of the code and without introducing huge security risks?

my $path = $ENV{'PATH'};
$ENV{'PATH'} = undef;
foreach my $p(split /:/, $path) {
    if ($p =~ m!^(/(usr|bin).*)!) {
        $ENV{'PATH'} .= ':' if $ENV{'PATH'};
        $ENV{'PATH'} .= $1;
    }
}
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