I don't have sources for Perl4, but I do not see any reason why any libraries would be involved. Most of what pack & unpack do is bit and byte manipulations.

If you were using Perl5, I would suggest using perl -V to look for build/configuration changes, but never having used Perl4, I am not even sure if it had this switch.

Another possibility that would be revealing (in Perl5 at least), would be to dump and compare the key/value pairs of %Config. Here I've picked out a few of the possibly relevant ones:

perl -MConfig -wle" print $k, ' => ', defined $v ? $v : '[n/a]' while 
+($k,$v) = each %Config"

archlibexp => c:\Perl\lib
archname => MSWin32-x86-multi-thread
cc => cl
ccflags => -nologo -Gf -W3 -MD -Zi -DNDEBUG -O1 -DWIN32 -D_CONSOLE -
cppflags => -DWIN32
dlsrc => dl_win32.xs
dynamic_ext => Storable Encode XS/Typemap MIME/Base64 IO B/C Devel/P
...
byteorder => 1234
...
i16size => 2
i16type => short
i32size => 4
i32type => long
i64size => 8
i64type => __int64
i8size => 1
i8type => char
...
intsize => 4
issymlink =>
ivdformat => "ld"
ivsize => 4
ivtype => long
...
lddlflags => -dll -nologo -nodefaultlib -debug -opt:ref,icf  -libpath:
+"c:\Perl\lib\CORE"  -machine:x86
ldflags => -nologo -nodefaultlib -debug -opt:ref,icf  -libpath:"c:\Per
+l\lib\CORE"  -machine:x86
...
loclibpth => /usr/local/lib /opt/local/lib /usr/gnu/lib /opt/gnu/lib /
+usr/GNU/lib /opt/GNU/lib
longdblsize => 10
longlongsize => 8
...
perllibs =>   oldnames.lib kernel32.lib user32.lib gdi32.lib winspool.
+lib ...
...
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