As you don't show how you have combined the two it is difficult to judge. This seems fairly straightforward and not too clumsy. I'm not sure how that might be condensed further.

$ perl -E '
> $str = q{ABCDEFGHIJK};
> substr( $str, 3, 3 ) =~ tr{A-Z}{a-z};
> say $str;'
ABCdefGHIJK
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I hope this is helpful.

This is neat, but was certainly not obvious. Maybe it is worth reminding that indeed substr can be used as a lvalue, as explicitely said in its perldoc:

You can use the substr() function as an lvalue, in which case EXPR must itself be an lvalue. If you assign something shorter than LENGTH, the string will shrink, and if you assign something longer than LENGTH, the string will grow to accommodate it. To keep the string the same length, you may need to pad or chop your value using "sprintf".

as explicitely said in its perldoc:

Said and shown :)

my $name = 'fred';
substr($name, 4) = 'dy';         # $name is now 'freddy'
my $null = substr $name, 6, 2;   # returns "" (no warning)
my $oops = substr $name, 7;      # returns undef, with warning
substr($name, 7) = 'gap';        # raises an exception
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Note that the lvalue returned by the three-argument version of substr() acts as a 'magic bullet'; each time it is assigned to, it remembers which part of the original string is being modified; for example:

$x = '1234';
for (substr($x,1,2)) {
    $_ = 'a';   print $x,"\n";    # prints 1a4
    $_ = 'xyz'; print $x,"\n";    # prints 1xyz4
    $x = '56789';
    $_ = 'pq';  print $x,"\n";    # prints 5pq9
}
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Wow, I can copy/paste :)