In the most simple case, you could just include the file using require. Unless you tell Perl otherwise (using a package ... declaration), everything will end up in the same namespace (i.e. main::). So yes, the variable $hello will be known within the included routine.

routines.pl

sub hello {
    print $hello;
}

1; # need to end with a true value
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mainscript.pl

#!/usr/bin/perl

require "./routines.pl";  # assuming it's in the current directory

$hello="hello123";
hello();                  # prints "hello123"
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Having said that, do use modules instead (with their own separate namespaces) for anything larger. This will help keep the mess to a minimum, which would otherwise ensue when the code grows... Same holds for global variables (like your $hello), so try to make judicious use of them...   In other words, you'd more typically pass $hello as an argument to the function, e.g.

sub hello {
    my $hello_msg = shift;
    print $hello_msg;
}

# ...

$hello="hello123";
hello($hello);
# or just:
# hello("hello123");
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