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;
}

[download]

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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In reply to Re: How to match previous line by moritz
in thread How to match previous line by qnguyen

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