⭐ in reply to How do I insert characters at certain positions in a string?

If you do not want to replace the character you can set the substring length to zero. 
my $padded = 'ABCDEFG';
substr($padded, 2, 0) = '1';
substr($padded, 1, 0) = '2';

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This will result in 'A2B1CDEFG'

In Section Seekers of Perl Wisdom