If you are writing these subroutines yourself, you should just write to a filehandle. open it somewhere and then just

print FH $whatever;
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If these are subroutines that someone else wrote, and you really don't want to edit them, you could do something like

open(FH, "output.txt");
my $oldfh = select(FH);
sub_that_writes_to_file();
select $oldfh;              # reset to whatever it was
close FH;
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This will work provided that the sub doesn't explicitly do print STDOUT.