Going through your log file with a simple while loop could do this, where any match is attached to a string variable. Check the code below

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

my $msg      = "";
my $log_file = "Output.txt";
open my $fh, '<', $log_file or die "can't open file: $!";
while ( my $line = <$fh> ) {
    chomp $line;
    if ( $line =~ /^\bstart.+?/i ) {
        $msg .= $line. ' ';
    }
    elsif ( $line =~ /^\bresponse.+?$/i ) {
        $msg .= $line. $/;
    }
    else { next; }
}
close $fh or die "can't close file:$!";

print $msg;
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