in reply to How to match previous line

You need something along the lines of 
use strict;
use warnings;
my $previous;
while (my $line = <$handle>) {
    if (defined($previous) && $line =~ /BEEP/ ) {
         print $previous;
    }
    $previous = $line;
}

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Of course you need additional code to extract the number from $previous, but that doesn't seem to be a problem for you.

Update: as ambrus++ pointed out in the CB this is a good place to use a continue block for assigning to $previous.

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Re^2: How to match previous line
by JavaFan (Canon) on Aug 26, 2009 at 16:33 UTC
    Considering that $previous is defined most of the time, it's better to check $line =~ /BEEP/ first. Or just eliminate the check for definedness all together: 
    my $previous = "";
while (<$handle>) {
    print $previous if /BEEP/;
    $previous = $_;
}

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    If "BEEP" is on the first line, it just prints nothing. Which is the same effect as not printing (assuming no assignment to $\ has been done).
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