How about this (untested):

open LOG, '<', 'somelog' or die $!;

my %tracker;
my $line;
while ($line = <LOG>)
{
  push @{ $tracker{$line} }, $.;
}
close LOG;

for my $key (keys %tracker) {
    print "Lines ", join(', ', @{$tracker{$key}}), " are the same.\n"
      if (@{ $tracker{$key} } > 1);
}
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