If you open a file using a filehandle, you can create a reference to it (by creating a reference to the typeglob) and then pass the reference to subs:

open( FH, '>', $filename ) or die $!;
my $ref2fh = \*FH;

foo( $ref2fh );
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My preferred method, though, is to open the file using a lexical variable (which eliminates the typeglob business):

open( my $fh, '>', $filename ) or die $!;

foo( $fh );
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See the section entitled "File and directory handles can be autovivified" in perldelta for perl 5.6 for more info on opening files using scalar variables (perl56delta on CPAN). This info is also in perlopentut under "Indirect Filehandles".

HTH

Update: added a link for perldelta