# ls -1 /nofile1 /nofile2  /tmp  2> >(while read line;do echo "my testing prefix: $line";done)
/tmp:
hsperfdata_oracle
kde-oracle
ksocket-oracle
vmware-root
my testing prefix: ls: cannot access /nofile1: No such file or directory
my testing prefix: ls: cannot access /nofile2: No such file or directory

$ sed 's/^/myprefix_/' a.txt
myprefix_aaa
myprefix_bbb
myprefix_ccc
myprefix_ddd

$ awk '$0="myprefix_"$0' a.txt
myprefix_aaa
myprefix_bbb
myprefix_ccc
myprefix_ddd

$ prefix="myprefix_"
$ awk -v prefix="$prefix" '$0=prefix$0' a.txt
myprefix_aaa
myprefix_bbb
myprefix_ccc
myprefix_ddd

$ nl -s "prefix_" a.txt | cut -c7-
prefix_aaa
prefix_bbb
prefix_ccc
prefix_ddd

paste -d'' <(yes prefix_) a.txt | head -n $(wc -l a.txt)

paste -d '\0' <(yes prefix_) a.txt | head -n $(wc -l < a.txt)