You can use a BEGIN block to operate on @ARGV (the $v=shift below, shifting off the first arg) before the implicit while (<>) {} loop (behind -n) takes effect treating the remaining arguments as file names.

$ perl -n -a -e 'BEGIN {$v=shift} if (($_ = "$F[1]\n") =~ /$v/) { prin
+t }' Vet TEST.txt
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