Just use a regular expression to modify the first character.

my $oldfilename='Testfile';
my $newfilename=$oldfilename;
$newfilename=~s/^./B/;
#
# another option instead of the regular expression would
# be to assign to a substr... This will probably be slightly
# faster
#
# substr($newfilename,0,1)='B';
#
link $oldfilename, $newfilename or die "link error $!";
unlink $oldfilename or die "unlink error $!";
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note the "link/unlink" that I do will not work under NT