use strict;
use warnings;

my $fileStr = <<STR;
F001            1.2
F101            3.2
solvent1      0
solvent2     3

F001            2.2
F101            7.2
solvent1      5
solvent2     0
STR

open my $fIn, '<', $fileStr or die "Couldn't open \$fileStr: $!\n";

##</code><code>##

# Look for the empty line between records
local  $/ = "\n\n";

while (defined (my $record = <$fIn>)) {

##</code><code>##

    my %recordData = map{split /\s+/, $_} grep {length $_} split "\n", $record;
    my @solvents = grep {/^solvent\d+/} keys %recordData;
    my @fractions = grep {/^F\d+/} keys %recordData;

##</code><code>##

    my ($zeroSolvent) = grep {!$recordData{$_}} @solvents; 

##</code><code>##

    
    print "${zeroSolvent}_$_ => $recordData{$_}\n" for @fractions;
}

##</code><code>##

solvent1_F101 => 3.2
solvent1_F001 => 1.2
solvent2_F101 => 7.2
solvent2_F001 => 2.2

##</code><code>##

use strict;
use warnings;

my $fileStr = <<STR;
F001            1.2
F101            3.2
solvent1      0
solvent2     3

F001            2.2
F101            7.2
solvent1      5
solvent2     0
STR

open my $fIn, '<', \$fileStr or die "Couldn't open \$fileStr: $!\n";

# Look for the empty line between records
local  $/ = "\n\n";

while (defined (my $record = <$fIn>)) {
    my %recordData = map{split /\s+/, $_} grep {length $_} split "\n", $record;
    my @solvents = grep {/^solvent\d+/} keys %recordData;
    my @fractions = grep {/^F\d+/} keys %recordData;
    my ($zeroSolvent) = grep {!$recordData{$_}} @solvents; 
    
    print "${zeroSolvent}_$_ => $recordData{$_}\n" for @fractions;
}