There are several examples given in the documentation for sort. The trick is specifying which fields of $a and $b to compare:

my @output;
while (<INPUT_FILE>)
{
    push @output, [ split(',', $_, 4) ];
}
my @sorted = sort {
    $a->[0] cmp $b->[0]
    ||
    $a->[3] <=> $b->[3]
} @output;
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I took the liberty of fixing the name of @outputRef (which is not a reference) and pushing each row as an anonymous array. I also use numeric comparisons on the phone numbers.