This is a FAQ (see perlfaq4):

perldoc -q count
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use warnings;
use strict;

my $str = "thisthis";
if ( my $n = () = ($str =~ m/this/g) ) {
    print qq{Found $n occurrence(s) of "this"\n};
}

__END__

Found 2 occurrence(s) of "this"
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Perl Idioms Explained - my $count = () = /.../g