for (qw(5234 523 52)) 
{
    print append($_ , '_'), "\n";
}

sub append 
{
    my ($val, $character) = @_;
    my @digits = split(//,$val);
    my $string = join ($character,@digits);
    $string =~ s/$character(\d)$/$1/ if @digits >= 4;
    return $string;
}