How to truncate STDIN line length?

Pipe output to:

cut -b 1-LIMIT

Where LIMIT is the desired line width.

Another tactic I use for viewing log files with very long lines is to pipe the file to "less -S". The -S option for less will print lines without wrapping, and you can view the hidden part of long lines by pressing the right-arrow key.

Not exactly answering the question, but if you want to stick with Perl and use a one-liner, a possibility is:

$ perl -pe's/(?<=.{25}).*//' filename

where 25 is the desired line length.