The "old school" way of doing this was by juggling the current default filehandle with select(), e.g.:

my $oldfh = select STDOUT;
my $out = IO::File->new('output.txt', 'w',) or die $!;
select $out;

# Do something involving print().  It will go to output.txt.

# Be nice and give back STDOUT
select $oldfh;
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This functionality is nicely wrapped in the oft-forgotten SelectSaver module (part of the Perl core distribution).

NB - don't confuse this version of select() with the four-argument version, which reports whether a file descriptor is ready for read/write.