I guess that your original text is UTF-8 encoded and you should decode it to Perl characters before performing regex substitutions. The character μ happens to be one of those where the UTF-8 encoding 0xc2 0xb5 contains the ISO-8859-1 encoding 0xb5. So, if you run s/(\x{B5}|\x{BC})/u/g; on UTF-8 encoded µ, then Perl replaces the 0xb5 as you asked for, leaving the 0xc2. This single character 0xc2 is not valid UTF-8, if you print it on a terminal which expects UTF-8 then you get the unicode replacement character.

use 5.020;
use Encode;
sub show {
    my ($char) = @_;
    printf "%s = %vx\n",encode('UTF-8',$char),$_[0];
}

my $emu = "\xc2\xb5";
show $emu;                      # µ = c2.b5

my $mu = decode('UTF-8',$emu);
show $mu;                       # µ = b5

$emu =~ s/(\x{B5}|\x{BC})/u/g; # Don't do this!
show $emu;                     # Âu = c2.75

$mu =~ s/(\x{B5}|\x{BC})/u/g;  # That's how to do it
show $mu;                      # u = 75

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In reply to Re: Substituting unicode character leaves special block by haj
in thread Substituting unicode character leaves special block by Anonymous Monk

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