prematch is just a wordier name for $`

$PREMATCH is the same as $` but it's not the same as ${^PREMATCH}:

* $PREMATCH
* $`

The string preceding whatever was matched by the last
successful pattern match, not counting any matches hidden
within a BLOCK or eval enclosed by the current BLOCK.

The use of this variable anywhere in a program imposes a
considerable performance penalty on all regular expression
matches. To avoid this penalty, you can extract the same
substring by using @-. Starting with Perl v5.10.0, you can
use the /p match flag and the ${^PREMATCH} variable to do
the same thing for particular match operations.

[download]

Source: perlvar.

In reply to Re^4: Best practice for reading delimited file by Kenosis
in thread Best practice for reading delimited file by rjbioinf

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