You are combining two unrelated options.

What do you expect -lf to produce???

If anything this undefined behaviour should be rejected.

Anyway my best guess is that

• -f is always (only) searching those two docs for snippets
• -l is always listing the full path of parsed pods

So this "anomaly" makes IMHO perfect sense.

Edit

Usage demo

$ perldoc -l perldoc
/data/data/com.termux/files/usr/lib/perl5/5.40.3/pod/perldoc.pod
$ perldoc -l Data::Dumper
/data/data/com.termux/files/usr/lib/perl5/5.40.3/aarch64-android/Data/
+Dumper.pm
$ perldoc -f x
    x
    xor     These operators are documented in perlop.

$
[download]

Update

If your intention was to find every perldoc documenting a certain keyword :

I once wrote a script which will parse them all for X<keyword> tags.

See Perldoc Keyword Search (update3)