#! /usr/bin/perl
use strict;
use warnings;
use List::Util qw( sum );

my @values;

while ( my $line = <DATA> ) {
    if ( $line =~ m{^start$} ... $line =~ m{^end$} ) {
        
        if ( $line !~ m{^(?:start|end)$} ) {
            push @values, $line;
        }
        elsif ( $line =~ m{^end$} ) {
            # einfacher Mittelwert
            my $average = sum(@values)/@values;
            print $average, "\n";

            # reset for next dataset
            @values = ();
        }

    }

}

__DATA__
start
0.23
0.32434
0.234
1.0
end
start
0.3423
1.4
end
final