# we are in bash, all is fine
$ eval `perl -le 'print $ENV{SHELL} =~ /csh/ ? "setenv FOO bar" : "export FOO=bar"'`

# now we switch to tcsh
$ tcsh
$ eval `perl -le 'print $ENV{SHELL} =~ /csh/ ? "setenv FOO bar" : "export FOO=bar"'`
$ export: Command not found.
$ echo $SHELL
/bin/bash