I think you want to do the same as in your shell script:

use strict;
use Cwd;
my $pwd = getcwd;
my $i = 'some_sub_directory';
my $new_path = "$pwd/$i";
$ENV{LSF_PATH} = "$new_path:$ENV{LSF_PATH}";
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If you have more than one directory you want to prepend to $ENV{LSF_PATH}, you can use the join function:

my @new_directories = map { "$pwd/$_" } 
    for ('some_dir', 'another_dir');
$ENV{LSF_PATH} = join ":", @new_directories, $ENV{LSF_PATH};
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