There's a global variable (though in recent versions of Perl it can be made more local on request) in Perl called $_. Many Perl functions, operators and looping constructs operate on $_ if you don't give them another variable to operate on instead.

For example, rather than:

foreach my $item (@array)
{
  print($item);
}
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You can take advantage of the fact that the default name used by foreach is $_, and the default thing printed by print is $_, and instead write:

for (@array)  # note "for" and "foreach" are synonyms
{
  print;
}
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Or just:

print for @array;
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Applying the same logic to your loop, the for is assigning each item in @array to $_; and if /$email/ is shorthand for if $_ =~ /email/; etc.