Enumerate substitutions with sed or awk

Question

Given the plain text file with lines

bli foo bla
 abc
 dfg
bli foo bla
 hik
 lmn

what sed or awk magic transforms it to

bli foo_01 bla
 abc
 dfg
bli foo_02 bla
 hik
 lmn

so that every occurence of 'foo' is replaced by 'foo_[occurence number]'.

Answer 1

awk '!/foo/||sub(/foo/,"&_"++_)' infile

Use gawk, nawk or /usr/xpg4/bin/awk on Solaris.