LanX's use of the tr/// operator (see perlop) is best (simplest, fastest) for simple counting.

For counting in cases of more complex matching, m// (perlop) can be used:

Win8 Strawberry 5.8.9.5 (32)  Sun 03/14/2021 17:12:13
C:\@Work\Perl\monks
>perl -Mstrict -Mwarnings

my $s = '1010011 f1e f0o g0e';

my $n =()= $s =~ m{ (?<= [fg]) [01] (?= [oe]) }xmsg;
print "$n [01] in '$s' \n";

^Z
3 [01] in '1010011 f1e f0o g0e'
[download]

See goatse (don't ask) for a discussion of the =()= "operator."