I strongly suggest using either File::Spec or it's nicer wrapper Path::Class for manipulating file paths in a portable way. You'll save yourself a lot of headaches.

use Path::Class;
my $file = file($filepath,"test.txt");
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Doing it the hard way, you can recombine a split using join. Alternatively, you could change the slashes using a regex substitution and then append as normal.

$filepath =~ s{/}{\}g;
$file = $filepath . "\test.txt";
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