You can just M-x `perl-mode` to activate perl mode.

Or, set a easy key to perl-mode. For example,

    (global-set-key (kbd "<f8>") 'perl-mode)

Or, you can put add this comment to the first line in the bash file

    # -*- mode: perl -*-

then, it'll open with perl mode.

Or, you can put the following at the bottom

    # Local Variables:
    # eval: (perl-mode 1)
    # End:

Does that answer your question?