That doesn't always provide the correct results.

my $string = "XXXX";
my @count = split( "XX", $string );
print $#count;   # Prints -1
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One could fix that by giving split third argument.

But it doesn't scale to generic patterns. Suppose you want to count how after characters are repeated (so, count not only "XX", but also "AA", "22", "--", etc). A pattern /(.)\1/ matches that, but

my $string = "XXYY";
my @count = split( /(.)\1/, $string, -1 );
print $#count;   # Prints 4, not 2.
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The reason is that if the pattern in contains capturing parenthesis, split returns what's captured in the parens as well.