Question:

Do you have long, and different rows:

searchme1|data1|more|auxiliary|data|that|is|very|long
searchmetoo|and|some|parametric|data|that|can|be|discarded
thirdsearch|and|the|file|looks|like|this?
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Where the first item after a separator "|" is the search term you need to capture and ignore the rest?

And somehow, this does not work or takes too long due to the long lines:

perl -ne 'print unless /search_this_text/' lines.txt
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Final question: What is the "modification" you need on that file? Mark a certain field, make the line start with #, remove the full line from the big file? (i.e. Will you require the same space, or will filesize change after such edit?)