But why tell him to read a filename at all in the case? I find that if one is taking filenames on the commandline, it's much better to use the diamond operator and let perl decide whether I should be reading from a bunch of files or STDIN, which allows for more flexibility.

And some technical critique - you didn't guard your print $last. This is what I have in mind: 
#!/usr/bin/perl
use strict;
use warnings;

my $prevline;
while (<>) {
    if(/ALARM:/) {
        print $. - 1, ": $prevline" if defined $prevline;
        print "$.: $_";
    }
    $prevline = $_;
}

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Makeshifts last the longest.

In reply to Re^2: when reading a file how to print a line one above the current line?? by Aristotle
in thread when reading a file how to print a line one above the current line?? by Anonymous Monk

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