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my @records;
while (<IN>) {
...
    # Stick your variables into an array for later use
    push @records, [ $NUM, $SEC, $LONG, $LAT ];
}

# Sort by LONG (index 0=NUM, 1=SEC, 2=LONG, 3=LAT)
my @sorted_recs = sort { $$records[$a][2] <=> $$records[$b][2] } @reco
+rds;

for my $rec (@sorted_recs) {
   # pull the variables from the current record
...

   print "Num=$NUM, Sec=$SEC, Long=$LONG, Lat=$LAT\n";
}