This is a very strange bug. It appears to be happening because the replacement is coming from an array; witness the following code:

#/usr/bin/perl -w
require 5.8.0;
use strict;

my($a1, $d1) = ("\x{00E0}", "\x{00E4}");
my($a2, $d2) = ("\x{0430}", "\x{0434}");
my($a3, $d3) = (["\x{0430}"], ["\x{0434}"]);
my @a4 = "\x{0430}"; my @d4 = "\x{0434}";

for (\&t2, \&t3, \&t4, \&t5) {
    my $text = $d1.$a1;
    warn "Before = ", join('.', unpack ("U*", ${text})), "\n\n";
    &$_($text);
    warn "After = ", join('.', unpack ("U*", ${text})), "\n\n";
}

sub t2 {
    $_[0] =~ s/$d1/$d2/g;
    $_[0] =~ s/$a1/$a2/g;
}

sub t3 {
    $_[0] =~ s/$d1/$d3->[0]/g;
    $_[0] =~ s/$a1/$a3->[0]/g;
}

sub t4 {
    $_[0] =~ s/$d1/$d4[0]/g;
    $_[0] =~ s/$a1/$a4[0]/g;
}

sub t5 {
    my $a5 = $a4[0];
    my $d5 = $d4[0];
    $_[0] =~ s/$d1/$d5/g;
    $_[0] =~ s/$a1/$a5/g;
}
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The t3() and t4() calls fail for me under perl-5.8.0 and with recent development sources at patchlevel 18736. The very latest development sources (@18777) succeed for all four cases, so this has clearly been fixed by a very recent patch.

The success of t5() in the above code suggests a workaround - grab the replacement variable into a scalar variable, and use that scalar for the replacement.

Hugo