If I'm understanding your issue, you have two data sets. One contained in removelist.txt:

user1@example.com
user3@example.com
...
[download]

The other in masterlist.txt:

user1@example.com
user2@example.com
user3@example.com
USER1@EXAMPLE.COM<mailto:USER1@EXAMPLE.COM>
user4@example.com
user5@example.com
user5@example.com<mailto:user5@example.com>
...
[download]

You want items from the first set to be removed from the second. Excellent suggestions have been offered, and this is a "classic issue."

See if the following will work for your situation:

use Modern::Perl;
use File::Slurp qw/ read_file write_file /;

my $removeList = join '|',
    map { chomp; $_ } read_file 'removelist.txt';
    
write_file 'finallist.txt',
    grep !/$removeList/i, read_file 'masterlist.txt';
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With the above two data sets, here is the output to finallist.txt:

user2@example.com
user4@example.com
user5@example.com
user5@example.com<mailto:user5@example.com>
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The script only does what you mentioned, viz., joins the removelist.txt items with the pipe character (alternation operator) for a !"or" regex of email addresses used by grep on the masterlist.txt lines.

Hope this helps!