You would use ... in the case where you could have the begin and end condition on the same line, but don't want that to be considered as having met the end condition.

my $save = tell( DATA );

while( <DATA> ) {
  print if /a/ .. /z/;
}

print "-" x 5, "\n";

seek( DATA, $save, 0 );

while( <DATA> ) {
  print if /a/ ... /z/;
}

exit 0

__DATA__
a b z
c d
w q
z
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