64 characters:

sub f{
map{/=>/?$b=$a:$a&&$a!~/=>/?push@{$x{$b}},$a:0;$a=$_}(@_,'');\%x
}
[download]

Update: 63 characters

sub f {
map{/=>/?$b=$a:$a&&$a!~/=>/&&push@{$x{$b}},$a;$a=$_}(@_,'');\%x
}
[download]

Update: 62 characters

sub f {
for(@_,''){/=>/?$b=$a:$a&&$a!~/=>/&&push@{$x{$b}},$a;$a=$_}\%x
}
[download]

-davisagli