Unless there is some dependancy you haven't disclosed -- say, for example, that the content of the file currently being processed determines which file to extract and process next -- your desire to extract them one at a time is not a war-stopper; it is merely one alternative approach:

Pseudocode:

un7zip ($archive);

@todo = readdir (extracted files);

for $todo(@todo) {
    process as you wish...
}
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