Do not enclose the variable to be expanded into single quotes: 
for i in 1 2 3 4 5 6 7 10 11 12 ; do
  perl -lane 'print p'$i'. "\t$_"' p$i.chr7.miRNA.txt
done

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In reply to Re: bash loop passing to perl command line by choroba
in thread bash loop passing to perl command line by ZWcarp

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