That's syntactically equivalent to a two-step operation:

my $qux = $foo;
$qux =~ s/bar/quux/;
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It just takes one line of code, so it's more idiomatic. There's absolutely nothing wrong with it, and I quite like it myself. There are only two things you might run into -- programmers who don't understand the idiom, and the looks of a my inside braces.

If it makes your life easier, go for it.