While I agree with chromatic in that dirtying your parameters is a bad idea, here's another solution for completeness

sub TimeStamp($) # Pass the stamp in as YYYYMMDD
{
    if( $_[0] =~ /(\d{4})(\d{2})(\d{2})/ )
    {   #returns in MMDDYYYY
        $_[0] = "$2/$3/$1";
    }
    else { warn "Invalid input to TimeStamp()" }
}
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This will change the variable that is passed to it as the @_ variable is really just a list of aliases (see. rinceWind's node for more info on the magicality of @_).
HTH

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broquaint