Your unshift syntax is incorrect. With the correct syntax, this technique works for me.

Without modifying @INC:

$ perl -Mstrict -Mwarnings -E '
require Const::Fast;                                                  
+                                             
say $Const::Fast::VERSION;
say $INC{q{Const/Fast.pm}};
'
0.011
/Users/ken/perl5/perlbrew/perls/perl-5.14.2_WITH_THREADS/lib/site_perl
+/5.14.2/Const/Fast.pm
[download]

Modifying @INC with the normal path:

$ perl -Mstrict -Mwarnings -E '
unshift @INC => q{/Users/ken/perl5/perlbrew/perls/perl-5.14.2_WITH_THR
+EADS/lib/site_perl/5.14.2};
require Const::Fast;
shift @INC;
say $Const::Fast::VERSION;
say $INC{q{Const/Fast.pm}};
'
0.011
/Users/ken/perl5/perlbrew/perls/perl-5.14.2_WITH_THREADS/lib/site_perl
+/5.14.2/Const/Fast.pm
[download]

Modifying @INC with a different path:

$ perl -Mstrict -Mwarnings -E '
unshift @INC => q{/Users/ken/perl5/perlbrew//perls/perl-5.14.2/lib/sit
+e_perl/5.14.2};
require Const::Fast;
shift @INC;
say $Const::Fast::VERSION;
say $INC{q{Const/Fast.pm}};
'
0.008
/Users/ken/perl5/perlbrew//perls/perl-5.14.2/lib/site_perl/5.14.2/Cons
+t/Fast.pm
[download]

Note how I've put @INC back to the way it was with shift after each require.

Choice of Const::Fast was completely arbitrary: I just happened to know I had different versions which was useful for demonstation purposes.