The $= variable holds the number of lines allowed on a page. 
When the line number exceeds this amount, write outputs a new page.
$= defaults to 60. 
The $- variable counts down as each line gets output on a page. 
When $- gets to 0, write outputs a new page. 
You can set $- to 0 to force a page output on the next call to write. 
Setting $- works only if you have a top-of-page format, e.g., FILEHANDLE_TOP. 


          
The following table lists the variables relating to pages and formats.
Variable     Usage
$=           Number of lines on a page; defaults to 60.
$-           Number of lines left on the page.
$%           Current page number.
$~           Name of format, defaults to same name as file handle.
$^           Name of top-of-page format, defaults to _TOP added to file handle.
$^L          String to output to advance page, defaults to formfeed character.