It doesn't work as you think!

An import method is defined in UNIVERSAL.pm, but that file is not loaded unless it is explicitly requested as use UNIVERSAL; which is very rare practice.

So, how does Perl handle the import method in modules not defining it?

It is a special case:

/* excerpt from gv.c in the perl source code */

if (!gv) {
    if (strEQ(name,"import") || strEQ(name,"unimport"))
        gv = MUTABLE_GV(&PL_sv_yes);
    else if (autoload)
        gv = gv_autoload_pvn(
        ostash, name, nend - name, GV_AUTOLOAD_ISMETHOD|flags
        );
    if (!gv && do_croak) {
        /* Right now this is exclusively for the benefit of S_method_c
+ommon
           in pp_hot.c  */
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when in a method call the requested method can not be resolved, perl checks if it is actually import (or unimport) and in that case, the call is just ignored.