I noticed that too and not sure if it is relevant, but I have been stung by something similar in the past. I would re-write the subroutine as:

my $type = event_hasher(\@event);

sub event_hasher {
    my $cur_event = shift;
    my %event_hash;
    for my $item (@$cur_event) {
        $item =~ s/://g;
        my ($key,$value) = split / /, $item;
        $event_hash{$key} = $value;
    }
    return \%event_hash;
}

$queue_changes{$event_id} = $type;
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Update:
As choroba points out below this doesn't solve the problem - in fact it creates it! You need to make a copy of the input - but you are doing that anyway so the aliasing is not the problem. Using choroba's test with your subroutine:

sub event_hasher {
    my @cur_event = @{$_[0]};
    my %event_hash;
    foreach(@cur_event){
        $_ =~ s/://g;
        my @place_holder = split / /, $_;
        $event_hash{$place_holder[0]} = $place_holder[1];
    }
    return %event_hash;
}

my @event = ('a:b c:d');
print "@event\n";
my %type = event_hasher(\@event);
print "@event\n";

# prints:
a:b c:d
a:b c:d
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