"join" is the opposite of "split".

my $path = join '\\', @dirs;
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You can append data to a variable using the concaternation operator '.'

$path = $path . '\\' . $file;
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You can also just use string interpolation.

$path = "$path\$file";
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But your original problem can probably be most easily solved with the tr/// operator.

$file =~ tr|/|\\|;
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