Similar to the external file approach, for simple tests, I've gotten some mileage out of just pointing STDIN to DATA.

use warnings;

*STDIN = *DATA;

print while <STDIN>;

__DATA__
Hello world
Once upon a time...
How now brown cow
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Another approach that doesn't need an external file is to use IO::String to create an in-memory file and alias STDIN to that. By directly changing the contents of the underlying scalar, you can control what will next be read from the in-memory file.