The difference is that a glob assignment doesn't set the "this has been imported" bit unless the importing is done via code that was compiled into a different package. See Re: How does strict determine if something is legally declared? (import) for more.

Just for reference, here's a comparison of the internals. Note the additional IMPORT( SV ) in FLAGS.

With package imp;.

C:\>perl -MDevel::Peek -e "BEGIN { require 'imp.pl'}; print Dump(*foo)
+"
SV = PVGV(0x1950c74) at 0x195aa84
  REFCNT = 2
  FLAGS = (GMG,SMG,MULTI,IN_PAD,IMPORT( SV ))
  IV = 0
  NV = 0
  MAGIC = 0x193716c
    MG_VIRTUAL = &PL_vtbl_glob
    MG_TYPE = PERL_MAGIC_glob(*)
    MG_OBJ = 0x195aa84
  NAME = "foo"
  NAMELEN = 3
  GvSTASH = 0x2260dc    "main"
  GP = 0x1950cc4
    SV = 0x19307cc
    REFCNT = 1
    IO = 0x0
    FORM = 0x0
    AV = 0x0
    HV = 0x0
    CV = 0x0
    CVGEN = 0x0
    GPFLAGS = 0x0
    LINE = 2
    FILE = "imp.pl"
    FLAGS = 0x1a
    EGV = 0x195aa84     "foo"
[download]

Without package imp;.

C:\>perl -MDevel::Peek -e "BEGIN { require 'imp.pl'}; print Dump(*foo)
+"
SV = PVGV(0x195064c) at 0x1930730
  REFCNT = 2
  FLAGS = (GMG,SMG,MULTI,IN_PAD)
  IV = 0
  NV = 0
  MAGIC = 0x1959054
    MG_VIRTUAL = &PL_vtbl_glob
    MG_TYPE = PERL_MAGIC_glob(*)
    MG_OBJ = 0x1930730
  NAME = "foo"
  NAMELEN = 3
  GvSTASH = 0x2260dc    "main"
  GP = 0x195ecf4
    SV = 0x19307cc
    REFCNT = 1
    IO = 0x0
    FORM = 0x0
    AV = 0x0
    HV = 0x0
    CV = 0x0
    CVGEN = 0x0
    GPFLAGS = 0x0
    LINE = 1
    FILE = "imp.pl"
    FLAGS = 0xa
    EGV = 0x1930730     "foo"
[download]

Thank you for that link, it describes exactly my problem. Well, apparently I have to accept this as "the way perl does things", even though I fail to see why it's sensible.