$ perl -e'while(1){printf"%d ",$i++;printf("field%d ",rand(20))for(0..rand(20));print"\n";}' >foo.dat
<CTRL-C>

$ head -4 foo.dat
0 field11 field6 field9 field14 field19 field11 field0 field2 field18 field10 field4 field17 field17 
1 field18 field12 field19 field9 field15 
2 field3 field16 field12 field17 field2 field10 field10 field1 field1 field10 field5 field1 field5 field11 field2 
3 field11 field2 field19 field16 field19 field15 field6 field10 field2 field7 field17 field8 field4 

$ ls -l foo.dat
-rw-------   1 notroot  14753792 Mar  6 13:13 foo.dat

$ time perl -lane '$n=shift@F;$f="@F";push@{$d{$f}},$n;END{for(sort keys %d){print"$_ -> @{$d{$_}}" if (@{$d{$_}}>1);}}' <foo.dat >foo.log

real       21.1
user       18.9
sys         0.9

$ tail -4 foo.log
field9 field9 field3 -> 50797 123541
field9 field9 field5 -> 25185 66389 134175 138790
field9 field9 field6 -> 8571 93213
field9 field9 field6 field2 -> 64192 151266