So what's in $term and which version of perl are you using? I can't reproduce your problem: 

use strict;
use warnings;
use Devel::Peek 'Dump';
my $token = "ba\x{f1}o";
utf8::upgrade($token);

Dump($token);
my $term = $token; # note: i have no clue what's in $term
print "match\n" if $token =~ /^$term/i;

[download]
update: output from that: 
SV = PV(0x814d988) at 0x814d524
  REFCNT = 1
  FLAGS = (PADBUSY,PADMY,POK,pPOK,UTF8)
  PV = 0x8160848 "ba\303\261o"\0 [UTF8 "ba\x{f1}o"]
  CUR = 5
  LEN = 6
match

[download]
update2: changed code so that $token and $term are the same. does not influence the outcome at all.

"What should it profit a man, if he should win a flame war, yet lose his cool?"

In reply to Re: Malformed UTF-8 by Joost
in thread Malformed UTF-8 by spiros

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