in reply to Converting HTML tags to upper case

When you are starting out with any language you can get a "blank screen" syndrome. The trick is to break the problem down into small chunks. I suppose I could write the script for you, but instead I'm going to suggest an approach you might like to take so you can begin learning more of the language.
You have a fairly clear spec on what you need, so start by taking that spec as a set of comments. Now "fill in the blanks" using your research and the online documentation, always remembering that "There's more than one way to do it": 
# prompt the user to enter the name of a HTML file. 
Use print, the read from STDIN

# The file will need verifying to check that the file  extenstion is 
# .html or .htm (upper case or lower case is fine). 
Use a regular expression or string comparison 

# Upon input of the validated name the file needs to be processed 
Open the file
Read each record in a loop

# and all lowercase tage, e.g (<html>) need converting to
# uppercase (<HTML>), 
# no other text should be converted 
You could craft a regular expression, but also checkout the many HTML 
+helper modules on CPAN

# also tag attribute values need to remain, 
# e.g (<img src="picture.jpg"> should convert to 
# <IMG SRC="picture.jpg"> and not <IMG SRC="PICTURE.JPG">. 
likewise

# At the end of the processing, the original file needs to be renamed
# with the .old extension instead of .htm or .html
# and the processed file should be given the original file name.
Generate the new filename, then use rename

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Now when you need more specific help with each section you can look up the online documentation and web resources, including SuperSearch.

In Section Seekers of Perl Wisdom