The first part of the trick is to 'slurp' the file. Set the special variable $/ to undef to read the entire file in one hit:

use strict;
use warnings;

my $file = <<FILE;
1
2
3


FILE

open my $fIn, '<', \$file;
my $str = do {local $/; <$fIn>};
close $fIn;

print "1-->\n$str<--1\n";
$str =~ s/\n+\Z//;
print "2-->\n$str<--2\n";
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Prints:

1-->
1
2
3


<--1
2-->
1
2
3<--2
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