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

##</code><code>##

for i in 1 2 3 4 5 6 7 10 11 12 ; do
  perl -lane"print qq{p$i\\t\$_}" p$i.chr7.miRNA.txt
done

##</code><code>##

for i in 1 2 3 4 5 6 7 10 11 12 ; do
  perl -lane'BEGIN { $i = shift(@ARGV); }
    print "p$i\t$_"' $i p$i.chr7.miRNA.txt
done