#!/usr/bin/perl
use strict;
use warnings;
use Data::Dumper;

# Assuming these are [ in, out ]
my @travel_times = (
    [  1,  5 ],
    [  2,  5 ],
    [ 20, 15 ],
);

my %count;
foreach my $time (@travel_times) {
    $count{in }{ $time->[0] }++;
    $count{out}{ $time->[1] }++;
}

print Dumper \%count;