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

my @array = ("a1 b1", "a2 b2", "a3 b3", "a5 b5", "a4 b4");

my %hash;
foreach my $element (@array) {
    my ($key, $value) = split(/ /, $element, 2);
    $hash{$key} = $value;
}

print Dumper \%hash;

say $hash{'a5'};

__END__

$ perl test.pl
$VAR1 = {
          'a3' => 'b3',
          'a5' => 'b5',
          'a4' => 'b4',
          'a2' => 'b2',
          'a1' => 'b1'
        };
b5