foo
bar
baz
quux

$ mysub = "disk$data1:[path.to]file.txt"
$ perl -pe "s/baz/''mysub'/" file1.txt

foo
bar
disk:[path.to]file.txt
quux

$ perl -pe "s'baz'"'mysub'"'" file1.txt

$ perl -pe "s/baz/q(''mysub')/e" file1.txt

perl -pwe "s/baz/$ENV{mysub}/" file1.txt

$ mysub = "disk$data1:[path.to]file.txt"

$ mysub = "disk\$data1:[path.to]file.txt"

perl -pe'BEGIN { $r = shift(@ARGV) } s/baz/$r/' "$mysub" file1.txt

perl -pe "BEGIN { $r = shift(@ARGV) } s/baz/$r/" 'mysub' file1.txt

$ sed -e "s/baz/''mysub'/" file1.txt

$ mysub='disk$data1:[path.to]file.txt'
$ perl -pe 's/baz/'$mysub'/' FILE
foo
bar
disk$data1:[path.to]file.txt
quux