See open for details:

open my $fhFile, '<', 'theFile' or die $!;
my $slurpedFile;
{
    local $/;
    $slurpedFile = <$fhFile>;
}
close $fhFile;

## Supplying a reference to a scalar as the filename
## initiates the opening of a ram file.
open my $fhRam, '<', \$slurpedFile or die $!;

## Now file operations on $fhRam will read from the slurped scalar
my $firstLine = <$fhRam>;
...
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