The expensive recursion can be optimized away. Also, you might also want to specify the "&" prototype to also allow map-like syntax.

sub scanl(&@) {
    my ($f, @rv) = @_;
    $rv[$_] = $f->($rv[$_-1], $rv[$_])
       for 1..$#rv;
    return @rv;
}

my @rv = scanl \&divide, 64, 4,2,4;
   -or-
my @rv = scanl { $_[0] / $_[1] } 64, 4,2,4;
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Finally, you could use $a and $b instead of $_[0] and $_[1], but that would prevent the use of existing functions directly. If that's not a problem, using $a and $b can be done by assigning references to *$ap and *$bp after doing

my $pkg = caller();
my $ap = do { no strict 'refs'; \*{$pkg.'::a'} };  local *$ap;
my $bp = do { no strict 'refs'; \*{$pkg.'::b'} };  local *$bp;
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