Do you know that a reference to substr works as you described? 
my $string    = 'abcdefghijklmnopqrstuvwxyz';
my $substring = \substr $string, 10, 10;
$$substring   = uc $$substring;
print $string, "\n";

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لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ

In reply to Re: Is there a module for object-oriented substring handling/substitution? by choroba
in thread Is there a module for object-oriented substring handling/substitution? by smls

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