Well to be honest I don't really know if this is a "better" solution than yours, but it reads more like the way I think. The y/ /0/ statement is only there because the interpreter was giving me a warning about a bitwise_and with a " " (there may be a better way to do this with options to sprintf).

So, in the spitrit of TIMTOWTDI.

sub ReturnFileMask($) {
    my $Numeric=sprintf "%3o",(shift)&07777;
    my $Mask='-xw@r';
    my $Tmp="-";
    foreach(split //,$Numeric)
    {
    y/ /0/;
    $Tmp .= substr($Mask, $_ & 4, 1) . 
            substr($Mask, $_ & 2, 1) . 
        substr($Mask, $_ & 1, 1);
    }
    return $Tmp;
}
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or alternitively, my second attempt, which doesn't ponce about with the bit wise ands. I think I much prefer this:

sub ReturnFileMask($) {
    my $Numeric=sprintf "%3o",(shift)&07777;
    my $Tmp="-";
    foreach(split //,$Numeric)
    {
    y/ /0/;
    $Tmp .= substr('----rrrr', $_, 1) .
        substr('--ww--ww', $_, 1) . 
        substr('-x-x-x-x', $_, 1);
    }
    return $Tmp;
}
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Or, how about the really simple

sub ReturnFileMask($) {
    my $Tmp="-";
    foreach(split //,sprintf "%3o",(shift)&07777)
    {
    y/ /0/;
    $Tmp .= ("---", "--x", "-w-", "-wx", "r--", "r-x", "rw-", "rwx")[$
+_];
    }
    return $Tmp;
}
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Nuance

Baldrick, you wouldn't see a subtle plan if it painted itself purple and danced naked on top of a harpsichord, singing "Subtle plans are here again!"