One way to do it is to inline the file in a HEREDOC, the variables will be interpolated.

my $oo = 'oop';
my $oop = <<"POOP";

I like to say p$oo
Hello $oo

POOP

print $oop
__END__

I like to say poop
Hello oop
[download]

But that's just standard double-quotish interpolation.

What I had imagined from dHarry's reply was that a here-doc could somehow be set up to do both levels of interpolation at once, i.e., interpolate a string, and then interpolate things that look like scalars within the string interpolated in the first step.

After using a here-doc (but this could have been an ordinary double-quoted string) to do the file interpolation, the code below uses an eval in the usual way to do the second-level interpolation; it's that eval step that I had hoped to eliminate.

use warnings;
use strict;


my $foo = 'there';
my $bar = 'now';

my $lines = <<'LINES';  # no interpolation here
hello $foo
goodbye $bar
LINES

open my $fh, '<', \$lines or die "opening ref: $!";

local $/ = undef;  # file slurp mode

my $file_interpolation = <<"FILE_INTERPOLATION";
pre-slurp
@{[ <$fh> ]}
post-slurp
FILE_INTERPOLATION

# can eval() step be eliminated?
my $eval_interpolation = eval "qq{$file_interpolation}";

print "$file_interpolation \n";
print "$eval_interpolation \n";
[download]

Output:

pre-slurp
hello $foo
goodbye $bar

post-slurp

pre-slurp
hello there
goodbye now

post-slurp
[download]