How do I change some char.s of the already printed text string? I.e.having printed

This is really hard for me to understand.
Once you "print" something - it is gone, passed to some other level of software.

Adding something before your print is fine.

my $hello ="Hello, World\n";
print "[]$hello";
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If you are talking about a command line window, there are various ways to do this - maybe just move the cursor back to the line to be modified, delete that line and then "re-print" that line. This will happen so fast that the user will not see it.