I stand corrected.

However, you're still better off using an external sort, as it allows you to gather the multiple values for each key together without loading the entire dataset into memory. Using a fairly simple loop like this:

#! perl 
use strict;

my( $key, @array ) = split "\t", <>;
while( <> ) {
    chomp;
    my( $newKey, $value ) = split "\t";
    if( $newKey eq $key ) {
        push @array, $value;
        next;
    }
    else {
        # Process @array for $key      
        
        #...
        
        ## Remember the newKey
        $key = $nextKey;
        ## And the reset the array
        @array = $value;
    }
}
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And a command line like:

sort < unsortedFile | perl theScriptAbove
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Or just sort the file and then feed it to the script as separate steps:

sort < unsortedFile > sortedFile
perl theScriptAbove sortedFile
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