use String::Interpolate qw( interpolate );

...[snip]...

# Method 4: worked! (not global)
#my ($before, $after) = $regex =~ m{/(.*)/(.*)/};
#$filename = replace($filename, $before, sub {interpolate($after)},0);

# Method 5: worked! (global)
my ($before, $after) = $regex =~ m{/(.*)/(.*)/};
$filename = replace($filename, $before, sub {interpolate($after)},1);