You can simplify that a bit. Regexes operate on $_ by default, and when you assign the results to @_, it will overwrite anything that is there, so there is no need to initialise it. You can also combine the statements into 1

Updated code with corrections by animator

perl -ne '@_=/(\w{3,})/g  and print"$.:@_$/"' test.jpg
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That said, linenumbers don't make much sense in a binary file, and if you supply multiple files or a wildcard, the linenumbers will not reset to 1 for each new file.