This is not tested.

my $reader = YAML::ReadStream( $file_handle );

while ( my $document = $reader->() ) {
    ...
}

sub YAML::ReadStream {
    my $handle = shift;
    my $buffer;
    return sub {
        if ( eof $handle ) {
            undef $buffer;
            return undef;
        }
        else {
            return YAML::_ReadOneDocument( $handle, \ $buffer );
        }
    };
}
sub YAML::_ReadOneDocument {
    my $handle = shift;
    my $buffer = shift;

    while ( 1 ) {
        if ( eof $handle ) {
            return YAML::Load( $$buffer );
        }

        read $handle, $$buffer, 8192, length $$buffer;

        my $yaml_header_position = index $$buffer, "\n---", length( "-
+--" );
        if ( $yaml_header_position >= 0 ) {
            my $doc = substr $$buffer, 0, $yaml_header_position, '';
            return YAML::Load( $doc );
        }
    }
}
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