You can use the function read for reading of file content in the string variable:

my $file_cont; # define a variable for keeping of file content
my $fname = '/tmp/your_file';

open INPUT, $fname or die "Can't open file $fname: $!";
binmode INPUT; # If your file is binare
read INPUT, $file_cont, -s $fname;
close INPUT or die "Can't close file $fname: $!";
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Format of function  read is following:

       read FILEHANDLE,SCALAR,LENGTH,OFFSET
or
       read FILEHANDLE,SCALAR,LENGTH
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