use strict;
use warnings;
use diagnostics;
use Data::Dumper;

my %hash;
my $key;
my $value;

my @data = ("foo",1, "bar",2, "foo",3, "baz",4);

while ($key = shift (@data))
{
   $value   = shift (@data);

   # push the value on the array
   push @{$hash{$key}}, $value;
}

print Dumper \%hash;