substr is an lvalue,

my $N = 3;
$_ = 'zebra';

# uppercase 1st $N
substr($_, 0, $N) = uc substr $_, 0, $N;
# lowercase last 3
substr($_, -3)    = lc substr $_, -3;

print;
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That trick can be used with s///, tr///, or any other mutator as well.