/* performance hack: if filename is absolute and it's a standard
		 * module, don't bother warning */
#ifdef MACOS_TRADITIONAL
#   define LIB_COMPONENT ":lib:"
#else
#   define LIB_COMPONENT "/lib/"
#endif
		if (file
		    && PERL_FILE_IS_ABSOLUTE(file)
		    && (instr(file, LIB_COMPONENT) || instr(file, ".pm")))
		{
		    continue;
		}
		CopLINE_set(PL_curcop, GvLINE(gv));
#ifdef USE_ITHREADS
		CopFILE(PL_curcop) = (char *)file;	/* set for warning */
#else
		CopFILEGV(PL_curcop) = gv_fetchfile(file);
#endif
		Perl_warner(aTHX_ packWARN(WARN_ONCE),
			"Name \"%s::%s\" used only once: possible typo",
			HvNAME_get(stash), GvNAME(gv));
...

with PERL_FILE_IS_ABSOLUTE being defined as (for my platform):

#define PERL_FILE_IS_ABSOLUTE(f)        (*(f) == '/')