I agree with others that a path-processing module is the way to go here, but if you just gotta have a regex, here's another:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"print qq{perl version: $]};
 ;;
 my $path = '/abc/def/ghi';
 ;;
 local our @dirs;
 $path =~ m{ (\A (?: / [^/]+ \b)+) (?{ push @dirs, $^N }) (?!) }xms;
 ;;
 dd \@dirs;
"
perl version: 5.008009
["/abc/def/ghi", "/abc/def", "/abc"]
[download]

With Perl version 5.10+, the odd-looking  (?!) can become  (*FAIL) from Special Backtracking Control Verbs in perlre (but the compiler will optimize it to (*FAIL) anyway). With version 5.18+ (IIRC), the package-global array  @dirs can become a my array; that bug was fixed.

Update 1: On second thought, the  \b anchor is a bit too alnum-specific: make it  (?! [^/]) instead:
    $path =~ m{ (\A (?: / [^/]+ (?! [^/]))+) (?{ push @dirs, $^N }) (?!) }xms;

Update 2: And if you want to reverse the order of the pieces, use a lazy quantifier:

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"print qq{perl version: $]};
 ;;
 my $path = '/=abc/def/ghi=/---';
 ;;
 local our @dirs;
 $path =~ m{ (\A (?: / [^/]+ (?! [^/]))+?) (?{ push @dirs, $^N }) (?!)
+ }xms;
 ;;
 dd \@dirs;
"
perl version: 5.008009
["/=abc", "/=abc/def", "/=abc/def/ghi=", "/=abc/def/ghi=/---"]
[download]