#!/usr/bin/perl
use strict;
use warnings;

my $file = '/path/to/file.txt';

open IN, "<", $file or die "Could not open $file:$!\n";

my $wanted;
while (my $line = <IN>) {
    if ($line =~ /(All avg = \d+)/) {
        $wanted = $1;
        last;   # get rid of this if you need/expect to match multiple lines
    }
}