Yes, except for a precedence problem:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $x = 'abcdefghi';
 substr($x, 5, 3) =~ s/g/G/;
 print $x;
"
abcdefGhi
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Without some sort of scoping disambiguation, the s/g/G/ binds to 3 in
    substr $x,5,3 =~ s/g/G/;