If I understand you correctly, perhaps something like this is what you want:

use List::Util 'sum';

my @su_data = do {
    local @ARGV = glob "SU*";
    map( ( split /\t/ )[ 1 ], <> );
    };

my @mu_data = do {
    local @ARGV = glob "MU*";
    map( ( split /\t/ )[ 1 ], <> );
    };

print join( "\t", 'Text String', @su_data, sum( @su_data ) / @su_data,
+ @mu_data, sum( @mu_data ) / @mu_data ), "\n";
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