perl -pe 's/^(..)H/${1}1/; s/^(.{7})L/${1}0/;' input > output
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The output is different on line 4, it prints

HH1HHHHH
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Explanation:

• -p reads the file line by line; it prints the line after running the code on it
• s/REGEX/STRING/ is substitution, it replaces what's matched by the regular expression REGEX by STRING
• ^ matches beginning of a line
• . matches anything but newline
• () create a "capture group", you can use $1 or ${1} to refer to the first capture group.

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