Assuming that you want to exclude all directories above the lowest-level "working directory," so that for a path like
    C:\foo\bar\ABCxyz\baz\boff\ABCxyz\myStuff.txt
you want
    \ABCxyz\myStuff.txt
returned, and also making some assumptions about the whole  ABC thing, this might do the trick:

c:\@Work\Perl\monks>perl -wMstrict -le
"use List::Util qw(max);
 ;;
 my $rx_wordic = qr{ (?<= \\) ABC [^\\]* }xms;
 ;;
 my $maxlen = max map length,
 my @paths = qw(
   C:\somedirectory\anotherDirectory\ABCxyz\myFile.txt
   C:\somedirectory\anotherDirectory\ABCxyz\Logs\myLogs.txt
   C:\somedirectory\anotherDirectory\ABCxyz\Stuff\myStuff.txt
   C:\foo\bar\ABCxyz\baz\boff\ABCxyz\myStuff.txt
   C:\foo\bar\ABCxyz\baz\boff\ABCxyz\xyzzy.zy\a.b\myStuff.txt
   C:\myStuff.txt
   C:\totally\bogus\myStuff.txt
   );
 ;;
 for my $path (@paths) {
   printf qq{%-*s -> }, $maxlen+2, qq{'$path'};
   $path =~ s{ \A .* (?= $rx_wordic) }''xmsg;
   printf qq{%-s \n}, qq{'$path'};
   }
"
'C:\somedirectory\anotherDirectory\ABCxyz\myFile.txt'        -> 'ABCxy
+z\myFile.txt'
'C:\somedirectory\anotherDirectory\ABCxyz\Logs\myLogs.txt'   -> 'ABCxy
+z\Logs\myLogs.txt'
'C:\somedirectory\anotherDirectory\ABCxyz\Stuff\myStuff.txt' -> 'ABCxy
+z\Stuff\myStuff.txt'
'C:\foo\bar\ABCxyz\baz\boff\ABCxyz\myStuff.txt'              -> 'ABCxy
+z\myStuff.txt'
'C:\foo\bar\ABCxyz\baz\boff\ABCxyz\xyzzy.zy\a.b\myStuff.txt' -> 'ABCxy
+z\xyzzy.zy\a.b\myStuff.txt'
'C:\myStuff.txt'                                             -> 'C:\my
+Stuff.txt'
'C:\totally\bogus\myStuff.txt'                               -> 'C:\to
+tally\bogus\myStuff.txt'
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Update: Thanks to Laurent_R for making the point about avoiding a leading \ on processed paths (update: but this feature is easy to get rid of if you don't want it).