"In general, if you know what you want to keep, use a regexp. If you know what you want to get rid of, use split." From a PerlMonks' posting Understanding Split and Join.

Given that, here's another option:

use Modern::Perl;

say map {(split)[1]} <DATA>;

__DATA__
IF-abc/12 [Se/0/0] [Second data][third]
IF-abc/1 [Gi/0/0] [Second data ]
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Output:

[Se/0/0][Gi/0/0]
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Hope this helps!

Update: However, if you're set on a regex for capturing that data, try the following instead of the split above:

say /(\[.*?\])/ while <DATA>;
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Output:

[Se/0/0]
[Gi/0/0]
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