++ is a prefix operator of increment. First, an increment is done, and then a new value is returned.
my $x = 41; say ++$x; # 42
The increment operation is not limited to working only with numbers. It can also handle strings.
my $a = 'a'; say ++$a; # b
A practical example is to increment filenames containing numbers. The file extension will survive, and only the numerical part will be incremented.
my $f = "file001.txt"; ++$f; say $f; # file002.txt ++$f; say $f; # file003.txt