use strict;
use warnings;
my @probs =(0,0.1,0.53,0.51,0.59,0.67,0.2,0.04,0.05,0.56,0.89,0.75);
my @pos;
my @segment;
foreach (0..$#probs){
    no warnings 'uninitialized';
    my $start;
    if ($probs[$_] > 0.5 && $probs[$_+1] <= 0.5){
        push @pos, $_;
    } elsif ($probs[$_] <= 0.5 && $probs[$_+1] > 0.5) {
        push @pos, $_ + 1;
    } 
}

use Data::Dumper;
print Dumper \@pos;