Well, $/ is the input record seperator, its what things like <> use to determine when to iterate to the next record. Default is "\n". some examples:

A) echo 'blah;hi;blah;' |perl -e 'while(<>) { print "$_\n"; }'

vs.

B) echo 'blah;hi;blah;' |perl -e '$/=";";while(<>) { print "$_\n"; }'
[download]

A outputs:
blah;hi;blah;

B outputs:
blah;
hi;
blah;

If you're interessted in changing this value, you might also look at split() which is generally safer to use (especially if you're not using strict and warnings).

-brad..