I like to take advantage of perl's command-line switches, so I'd do something like this:

#!perl -pli

$_ .= '_' unless $. == 1;  # append underscore, except on first line
close ARGV if eof;         # reset line numbering at end of file
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-p loops over the input, reading it into $_ and printing it back out. -l handles line-endings. -i modifies files in place.